This present invention relates to novel memory devices. The invention is useful in the development, manufacture, and use of a variety of devices and/or technologies, including, inter alia, memory devices for electronic computers, associative memory systems, circuit elements with programmable resistance for creating synapses for neuronal nets, direct access data banks, and new types of video/audio equipment.
Modern electronic computers employ several different types of memory devices for various purposes and functions requiring different performance/operating characteristics, e.g., read/write and storage/retrieval speeds. The multiplicity of different requirements for the various memory devices substantially complicates the operation of computer systems, increases start-up times, and complicates data storage.
As a consequence of the above-mentioned drawbacks and disadvantages associated with current memory device technology, a high priority task of the microelectronics industry is creation/development of a universal memory device/system having high read/write speeds, high storage density, and long term data retention characteristics.
A number of electronic memory or switching devices have been proposed or developed which include a bi-stable element that can be controllably alternated between high impedance and low impedance states by application of an electrical input, e.g., a voltage equal to or greater than a threshold voltage. Memory and switching devices utilizing such threshold-type behavior have been demonstrated with both organic and inorganic thin film semiconductor materials, including amorphous silicon, chalcogenides such as arsenic trisulphide-silver (As2S3xe2x80x94Ag), organic materials, and heterostructures such as SrZrO3(0.2% Cr)/SrRuO3. See, for example: U.S. Pat. Nos. 5,761,115; 5,896,312; 5,914,893; 5,670,818; 5,770,885; and 6,150,705; Russian Patent No. 2,071,126; S. R. Ovshinsky, Phys. Rev. Lett., 36, 1469 (1976); J. H. Krieger, et al., J. Struct. Chem., 34, 966 (1993); J. H. Krieger, et al., Synthetic Metals, 122, 199 (2001); R. S. Potember, et al., Appl. Phys. Lett., 34 (6), 405 (1979); Y. Machida, et al., Jap. J. Appl. Phys., Part 1, 28 (2), 297 (1989); A. Beck, et al., Appl. Phys. Lett., 77, 139 (2000); and C. Rossel et al., J. Appl. Phys. (2001), in press.
U.S. Pat. No. 6,055,180 to Gudeson, et al. discloses an electrically addressable, passive storage device for registration, storage, and/or processing of data, comprising a functional medium in the form of a continuous or patterned structure capable of undergoing a physical or chemical change of state. The functional medium comprises individually addressable cells each of which represents a registered or detected value or is assigned a predetermined logical value. Each cell is sandwiched between an anode and cathode (electrode means) which contact the functional medium of the cell for electrical coupling therethrough, with the functional medium having a non-linear impedance characteristic, whereby the cell can be directly supplied with energy for effecting a change in the physical or chemical state in the cell.
A disadvantage/drawback of the storage device of U.S. Pat. No. 6,055,180, however, is that writing of information can occur only once, and reading of the stored information is performed optically, thereby increasing the size and complexity of the device and its use, at the same time reducing reliability of reading of the information due to the difficulty in accurately positioning the optical beam. In addition, an alternate writing method utilizing thermal breakdown caused by application of a high voltage is also disadvantageous in that writing of information can only occur once, and high voltages, hence high electrical fields, are required.
JP 62-260401 discloses a memory cell with a three-layer structure comprised of a pair of electrodes with a high temperature compound (i.e., molecule) sandwiched therebetween, which memory cell operates on a principle relying upon a change of electrical resistance of the compound upon application of an external electric field. Since the conductivity of the compound can be controllably altered between two very different levels, information in bit form can be stored therein.
U.S. Pat. No. 5,761,116 to Kozicki et al. discloses a xe2x80x9cprogrammable metallization cellxe2x80x9d comprised of a xe2x80x9cfast ion conductorxe2x80x9d, such as a film or layer of a chalcogenide doped with a metal ion, e.g., silver or copper, and a pair of electrodes, i.e., an anode (e.g., of silver) and a cathode (e.g., of aluminum), spaced apart at a set distance on the surface of the doped chalcogenide. The silver or copper ions can be caused to move through the chalcogenide film or layer under the influence of an electric field. Thus, when a voltage is applied between the anode and the cathode, a non-volatile metal dendrite (xe2x80x9cnano-wirexe2x80x9d) grows on the surface of the chalcogenide film or layer (xe2x80x9cfast ion conductorxe2x80x9d) from the cathode to the anode, significantly reducing the electrical resistance between the anode and cathode. The growth rate of the dendrite is a function of the applied voltage and the interval of its application. Dendrite growth may be terminated by removing the applied voltage and the dendrite may be retracted towards the cathode by reversing the polarity of the applied voltage.
U.S. Pat. No. 5,670,818 to Forouhi et al. discloses a read-only memory device in the form of an electrically programmable antifuse comprised of a layer of amorphous silicon between metal conductors. Under application of a high voltage, a portion of the amorphous silicon layer undergoes a phase change and atoms from the metal conductors migrate into the silicon layer, resulting in formation of a thin conducting filament (xe2x80x9cnano-wirexe2x80x9d) composed of a complex mixture of silicon and metal.
The principal shortcomings of the above-described memory devices relying upon nano-wire formation are related to the low operational speeds caused by the extended interval required for effecting substantial change in the electrical resistance between the electrodes/conductors and to the high voltage required, e.g., xcx9c60 V. Such drawbacks significantly limit practical use of the cells in current high speed electronic devices.
U.S. Pat. No. 4,652,894 to Potember et al. discloses a current-controlled, bi-stable threshold or memory switch, comprised of a layer of a polycrystalline metal-organic semiconductor material sandwiched between a pair of metallic electrodes, wherein the layer of metal-organic semiconductor material is an electron acceptor for providing fast switching at low voltages between high and low impedance states.
Practical implementation of the threshold memory switch of U.S. Pat. No. 4,652,894 is limited, however, principally due to the use of low temperature metal-organic semiconductor compounds which are not sufficiently mechanically robust, and more importantly, are insufficiently resistant to chemical degradation when subjected to the elevated temperatures commonly associated with modern semiconductor manufacturing processing, i.e., greater than about 150xc2x0 C. and as high as about 400xc2x0 C. In addition, the physical characteristics of the metal-organic semiconductor materials cause poor repeatability of the read/write/erase cycle, and storage is limited to only 1 bit of formation, thereby prohibiting use in high information density applications/devices.
In view of the above, there exists a clear need for memory devices which are free of the above-described shortcomings, drawbacks, and disadvantages associated with memory devices of the conventional art. The present invention, therefore, has as its principal aim the development of a universal memory device/system for high speed data storage and retrieval, with capability of long term storage at high bit densities.
An advantage of the present invention is an improved memory storage and retrieval device.
Another advantage of the present invention is an improved memory storage and retrieval device not requiring formation of conventional semiconductor junctions.
Yet another advantage of the present invention is an improved memory storage and retrieval device which can be readily fabricated from a variety of materials.
Still another advantage of the present invention is an improved memory storage and retrieval device having very high read and write speeds, long term data retention, and high data storage density.
Additional advantages and other features of the present invention will be set forth in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims.
According to an aspect of the present invention, the foregoing and other advantages are obtained in part by a memory storage and retrieval device, comprising:
(a) an electrically conductive first electrode;
(b) an electrically conductive second electrode; and
(c) a layer stack intermediate the first and second electrodes, the layer stack comprising:
(d) at least one active layer with variable electrical conductivity; and
(e) at least one passive layer comprised of a source material for varying the electrical conductivity of the at least one active layer upon application of an electrical potential difference between the first and second electrodes.
In accordance with embodiments of the present invention, the electrical conductivity of the at least one active layer is reversibly varied upon introduction and removal of charged species; and the at least one passive layer is comprised of a source material for reversibly donating the charged species to and accepting the charged species from the active layer.
According to preferred embodiments of the invention, the charged species comprise ions or a combination of ions and electrons, the ions being selected from the group consisting of: metal ions, metal-containing ions, non-metal ions, and non-metal-containing ions.
Embodiments of the present invention include those wherein the layer stack comprises a pair of active layers in mutual contact; and the layer stack may further comprise:
(f) at least one barrier layer comprised of a material which impedes spontaneous movement of the charged species when the electrical potential difference is not applied between the first and second electrodes.
Further embodiments of the present invention include those wherein the at least one barrier layer is positioned within the stack intermediate the active layer and the passive layer; embodiments wherein the layer stack comprises first and second active layers and the at least one barrier layer is positioned within the stack intermediate the first and second active layers; and embodiments wherein the layer stack comprises first and second passive layers in respective contact with the first and second electrically conductive electrodes.
According to still further embodiments of the present invention, the at least one active layer and the at least one passive layer are each comprised of the same material, e.g., an intercalation compound such as LixVSe2 or LixHfSe2, whereby the stack effectively comprises a single layer.
In accordance with embodiments of the present invention, each of the first and second electrically conductive electrodes comprises at least one electrically conductive material selected from the group consisting of metals, metal alloys, metal nitrides, oxides, sulfides, carbon, and polymers; and according to particular embodiments of the invention, each of the first and second electrically conductive electrodes comprises at least one material selected from the group consisting of aluminum (Al), silver (Ag), copper (Cu), titanium (Ti), tungsten (W), their alloys and nitrides, amorphous carbon, transparent oxides, transparent sulfides, and conductive organic polymers, each of the first and second electrically conductive electrodes being from about 3,000 to about 8,000 xc3x85 thick, preferably about 5,000 xc3x85 thick.
According to embodiments of the present invention, the at least one active layer comprises at least one material with a relatively lower intrinsic electrical conductivity when free of the charged species and a relatively higher electrical conductivity when doped with the charged species, i.e., the at least one active layer comprises at least one material selected from the group consisting of dielectrics, semiconductors, ferroelectrics, ceramics, organic polymers, molecular crystals, and composites thereof, wherein the at least one active layer includes a plurality of channels or pores extending therethrough for facilitating movement of the charged species therein, selected from the group consisting of Si, amorphous Si, organic polymers, silicon dioxide (SiO2), aluminum oxide (Al2O3), titanium dioxide (TiO2), boron nitride (BN), carbon tri-nitride (CN3), copper oxide (Cu2O), vanadium oxide (V2O3), ferroelectric materials, materials containing electrolyte clusters, and intercalation compounds selected from LixVSe2 and LixHfSe2; and the at least one active layer is from about 50 to about 1,000 xc3x85 thick, preferably about 100 xc3x85 thick.
In accordance with embodiments of the present invention, the at least one passive layer comprises at least one super-ionic conductor material or intercalation compound, wherein the at least one super-ionic conductor material or intercalation compound reversibly donates and accepts charged species; e.g., ions or ions+electrons, the ions being selected from the group consisting of silver (Ag), copper (Cu), gold (Au), lithium (Li), sodium (Na), potassium (K), zinc (Zn), magnesium (Mg), other metal or metal-containing ions, hydrogen (H), oxygen (O), fluorine (F), and other halogen-containing ions; and the at least one super-ionic conductor material or intercalation compound is selected from the group consisting of AgI, AgBr, Ag2S, Ag2Se, Ag2xe2x88x92xTe, RbAg4I5, CuI, CuBr, Cu2xe2x88x92xS, Cu2xe2x88x92xSe, Cu2xe2x88x92xTe, AgxCu2xe2x88x92xS, Cu3HgI4, Cu3HgI4, AuI, Au2S, Au2Se, Au2S3, NaxCuySe2, LiNiO2, LixTiS2, LixMoSe2, LixTaS2, LixVSe2, LixHfSe2, LixWO3, CuxWO3, NaxWO3, Naxcex2-Al2O3, (AgI)x(Ag2OnB2O3)1xe2x88x92x, Ag2CdI4, CuxPb1xe2x88x92xBr2xe2x88x92x, Li3M2(PO4)3xe2x80x94where M=Fe, Sc, or Cr, K3Nb3B2O12, K1xe2x88x92xTi1xe2x88x92xNbxOPO4, SrZr1xe2x88x92xYbxO3, Sr1xe2x88x92x/2Ti1xe2x88x92x, NbxO3xe2x88x92xcex4, xcex2-Mg3Bi2, Cs5H3(SO4)x.H2O, M3H(XO4)2xe2x80x94where M=Rb, Cs, or NH4 and X=Se or S, NaZr2(PO4)3, Na4.5FeP2O8(OF)1xe2x88x92x, ZrO2xe2x88x92x, CeO2xe2x88x92x, CaF2, and BaF2, and is from about 20 to about 100 xc3x85 thick, preferably about 50 xc3x85 thick.
According to embodiments of the present invention, the at least one barrier layer comprises at least one material selected from the group consisting of Li3N and LiAlF4 and is from about 20 to about 300 xc3x85 thick, preferably about 50 xc3x85 thick.
Additional advantages and aspects of the present invention will become readily apparent to those skilled in the art from the following description, wherein embodiments of the present invention are shown and described, simply by way of illustration of the best mode contemplated for practicing the present invention. As will be described, the present invention is capable of other and different embodiments, and its several details are susceptible of modification in various obvious respects, all without departing from the spirit of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as limitative.